A typical conventional projection display is disclosed, for example, in U.S. Pat. No. 4,040,047 entitled "Erasable Thermo-Optic Storage Display of a Transmitted Image" distributed on Aug. 2, 1977. This apparatus does not control the input power of an illumination light source in response to brightness of the screen. For this reason, screen brightness might drop due to the change with time of the illumination light source or a projection light source.
The prior art apparatus described above is not free from a drawbacks in that since brightness of the light source fluctuates due to the change with time of the projection light source and due to the change of ambient conditions, screen brightness is not constant and a stable screen image cannot be obtained. More specifically, brightness of a Xenon lamp (hereunder, Xe lamp), which is this kind, drops with the passage of time and the image becomes somewhat darker and difficult to watch with the passage of a few months after the start of use. Therefore, it has been customary to replace the Xe lamp by new one at this stage by judging that the life of the lamp is reached.
The prior art apparatus is not at all considered at this point but replaced the Xe lamp with a new one within a short period by judging that the service life is over. Therefore, the lamp is not utilized fully and a stable screen image in brightness cannot be obtained.